1. Field
he present invention relates generally to a wireless communications network with multiple, potentially mobile User Equipment (UEs) communicate with a group of base stations, and more specifically to multicast/broadcast communications in such a network.
2. Background
FIG. 1A is an illustration of an exemplary wireless communications network. A group of User Equipment (UE, one of which is labeled 20), some or all of which may be mobile, are in communication with a group of base stations (one of which is labeled 26). At any given moment each UE 20 communicates with a base station 26 using a certain channel (a, b, c, or d). As used herein a channel includes an arrangement of frequencies or communications links to facilitate communications. For example, in some systems a channel comprises two communication links, each using a separate frequency. In FIG. 1A the UE 20 may transmit to the base station 26 on a first frequency, and may receive from the base station 26 on a second frequency. This pair of communication links constitutes a channel. Each base station 26 may be configured to provide service on one or more channels and each UE 20 may be configured to receive service on one or more channels.
In order to optimize network resource usage, and provide the highest quality of service, the UEs 20 may be maintained in a substantially evenly distributed channel assignment configuration. That is, communication traffic load is substantially balanced among the available channels on the available base stations. In order to maintain the balanced load in the dynamic environment of frequent UE communication link establishment and conclusion and frequent changes in which base station is currently serving an individual mobile UE, each base station 26 is configured to broadcast certain network status information to the UEs and each UE is configured to monitor the network status information and to monitor communication link quality parameters and to choose channels and base stations based on the information and the parameters. The process of choosing a channel for service may be called channel selection, and the process of choosing a particular base station transceiver for service may be called cell selection or cell reselection.
Certain standards have been developed and are currently used for managing this dynamic selection and reselection process. Such a standard is 3GPP Release 1999 (R'99) cell reselection. In accordance with R'99 cell reselection, certain network status information is broadcast. For example, System Information Block (SIB) 3, SIB 4, SIB 11, and SIB 12 data is broadcast for use by UEs for cell reselection. Additionally, SIB data can be used to calculate certain cell parameters, such as Qoffsets,n, Qhysts, Qhcss, and Qmeass, which are also used for cell reselection. As part of channel and/or cell selection UEs may also perform channel quality testing according to an adopted standard such as a suitability test in the sense of 3GPP TS 25.304. In order to minimize unnecessary reselection activity, the R'99 reselection algorithm tends to favor the current channel and cell. This tendency is called stickiness.
For example, in the network of FIG. 1A, the UEs 20 are mobile phones with users initiating, receiving and terminating phone calls while driving. As an individual UE gets farther from a currently serving base station, the signal associated with the current channel becomes weaker. The UE monitors SIB data and performs a R'99 cell reselection. As part of the R'99 cell reselection the UE determines that a new channel being served by a new cell is suitable, and begins communicating on the new channel in the new serving cell. As the UEs reselect channels and cells, a desired distribution is maintained by the reselection algorithm. Because high activity tends to lower the quality of the channels and cells, and the algorithm prefers to select higher quality channels and cells, during reselection a UE is more likely to select a lower traffic channel and cell. Thus, the traffic in the network tends to be balanced.
In addition to two-way communication, the network shown in FIG. 1A may be used for broadcast services as well. One such service is a point-to-multipoint Multimedia Broadcast/Multicast Service (MBMS). In this type of broadcast UEs subscribed to the service receive a notice of an upcoming broadcast, say results of today's football matches, from the currently serving cell. The notice contains information as to which channel the broadcast will be on. The UEs whose user's who want to participated in the broadcast tune to the correct channel, and receive the broadcast information. A result of the broadcast is that a potentially large number of UEs are congregated on the same channel.
FIG. 1B is a diagram showing an exemplary group of UEs (one of which is labeled 20) congregated on a single channel a wirelessly receiving broadcast data from a group of base stations (one of which is labeled 26). During normal operation such a congregation would overload the network because the network does not have the capacity to service such a large group of UEs on a single channel. However, during the broadcast this congregation does not overload the network because each of the UEs may be receiving large amounts of data, but is transmitting only a minimal amount of data, and the broadcast data is concentrated on a single common channel rather than consuming separate resources for each UE in the congregation. The base stations 26 broadcast the data on the published channel, and each UE 20 receives the data on that channel.
After the end of the broadcast, however, the UEs will return to normal transmission levels, and the congregation concentrated on the single channel a may overload the network resources. This can be avoided by generally broadcasting a disperse command to the UEs, which respond to the disperse command by selecting a channel and a cell for service. The results of such a dispersion will be that the UEs move to different channels and camp on cells serving those channels with a distribution like that shown in FIG. 1A, where the UEs are distributed amongst the cells on various channels a, b, c, and d.
There is therefore a need in the art for UEs to be able to “disperse” across channels in response to a disperse command in an efficient and simple manner in such a way that the resulting load is balanced across channels and cells.